Modem integrated circuits often use in-situ doped silicon layers. The silicon layers may be either amorphous silicon or polysilicon. Such doped films are used extensively in Linear CMOS integrated circuits both as high quality resistors, and as the bottom plates for capacitors. In-situ doped polysilicon films are very attractive because, for example, thermal oxides grown on in-situ phosphorus doped polysilicon films are known to exhibit higher breakdown fields, and less leakage than similar oxides grown on ex-situ phosphorus doped polysilicon films (e.g., films doped at temperatures around 900-1000.degree. C. from PBr.sub.3 vapors). Also, it is possible to obtain lower polysilicon film resistivity on in-situ doped films than on ex-situ doped films because of the uniform and intimate mixing dopants with the silicon matrix. Typically, in-situ doped silicon layers are formed by combining silane and a dopant-containing gas, such as PH.sub.3 in a furnace.
FIG. 1 is a diagram useful in understanding some aspects of the current process. In FIG. 1, reaction furnace 11 is illustratively, a reaction furnace made by BTI, Incorporated, Bruce Technologies International Inc., 25 Esquire Rd., North Billerica, Mass. Reference numeral 13 denotes a heating apparatus which heats the interior of furnace 11. Reference numeral 23 denotes six-caged wafer boats, each boat being filled with 26 wafers loaded proximately back to back. A gas mixture is typically admitted to the furnace through opening 19 at the front of the furnace and through tube 15 which extends 44 inches into the center of the furnace and is positioned below the center of the wafer load (between boats 3 and 4) and also through tube 17 which extends approximately 34 inches into furnace 11. Tubes 19, 15, and 17 are each connected to gas cabinet 21 which supplies either nitrogen, silane, or phosphine through a series of valves and mass-flow controllers denoted by reference numeral 24.
Conventional processing sequences follow the teachings of Wonsowicz in U.S. Pat. No. 4,460,416 issued Jun. 17, 1984. Wonsowicz teaches that both the PH.sub.3 and silane gases should be turned on at the same time and gradually increased from zero to respective steady state values. After the layer of doped polysilicon has been formed, the system is purged with nitrogen. The nitrogenpurged step eliminates the hazardous gases from the system tubing.
Those concerned with the development of integrated circuit processing have continually sought better methods and apparatus for the formation of in-situ doped silicon films.